That’s a great question. WordPress doesn’t automatically serve retina (basically 2x the size) images based on the device or screen size or anything. But I know there is at least one WordPress plugin that will try to detect retina displays and then swap out the image with one that is 2x the size.

Basically if you are actively supporting retina screens then you would just want to set Imsanity max size limit to 2x whatever the size of images that you usually display on your site. So, for example, if you usually display images at 500×500 px then you would want to make sure Imsanity max was set to at least 1000×1000.